FIG. 13 is a schematic view of a conventional surface acoustic wave (SAW) filter device 501 disclosed in Japanese Patent Laid-Open Publication No. 2002-314371. The SAW filter device 501 includes an input port 2, input/output ports 3A and 3B, output ports 4A and 4B, and SAW filters 5 and 6 provided on a piezoelectric substrate 1. The SAW filter 5 is adapted to receive an unbalanced signal from the input port 2 and output balanced signals to the input/output ports 3A and 3B. The SAW filter 6 is adapted to receive balanced signals from the input/output ports 3A and 3B and output balanced signals to the output ports 4A and 4B.
The SAW filter 5 includes longitudinally-coupled surface acoustic wave (SAW) resonators 5A and 5E. The SAW resonator 5A includes an unbalanced signal input port 5B and balanced signal output ports 5C and 5D. The SAW resonator 5E includes an unbalanced signal input port 5F and balanced signal output ports 5G and 5H.
The balanced signal output ports 5C and 5D receive and output signals having the same phase, i.e. having the phase difference which is substantially zero degree, while the balanced signal output ports 5G and 5H receive and output signals having the same phase. The signals output from the balanced signal output ports 5C and 5D have phases which are different by substantially 180 degrees from the phases of the signals output from the balanced signal output ports 5G and 5H, that is, .which are reversed from opposite the phases of the signals output from the balanced signal output ports 5G and 5H, respectively.
The balanced signal output port 5C and the balanced signal output port 5D are connected to each other and electrically connected to the input/output port 3A. The balanced signal output port 5G and the balanced signal output port 5H are connected to each other and electrically connected to the input/output port 3B.
FIG. 14 illustrates propagation characteristics of the SAW filter 5 in the SAW filter device 501. In the SAW filter 5 located at the first stage, spurious S is generated in a high frequencies in of a predetermined pass band PB0, and increases an insertion loss of the SAW filter device 501.